When transmitting a data packet (frame) over a serialized channel, it is common to perform some kind of signal encoding prior to transmitting the data over the physical media. Some of the reasons for this encoding are as follows:
(a) To reduce the effective transmission rate by encoding the bit stream into symbols using a multi-level code. The resulting baud rate is thus reduced. PA1 (b) To be able to identify start of frame characters uniquely from the data by the use of additional codepoints to allow start and end frame delineation ("framing"). PA1 (c) To improve error robustness by the use of additional codepoints. PA1 (d) To enable the carrying of control data over the channel through the use of additional codepoints. PA1 (e) To be able to map data so as to obtain an overall DC balance on a character by character basis by the use of additional codepoints. PA1 (a) signal degradation due to cross-talk between transmitter and receiver at one end or between transmitters at one end when two or more cables are used for transmission, PA1 (b) signal attenuation due to lossy medium, and PA1 (c) limitation of signal transmit launch power due to Federal Communications Commission (FCC) compliance.
The use of unshielded twisted pair transmission lines in existing buildings for local area networks (IAN's) and in other areas, has created a need for a technique that will allow the transmission of high speed serial bit streams containing data frames. At present there is a need for a method of coding a 500 Mbps serial bit stream containing data frames into an efficient physical layer stream for subsequent transmission over a single pair of Unshielded Twisted Pair (UTP). For example, transmission of such a stream over a Category 5 cable (UTP-5) twisted pair up to a distance of 50 meters is desired.
There are numerous problems in trying to transmit data at these rates using only unshielded twisted pairs (UTP-5) as the medium. These may be summarized as follows:
Accordingly, it is an object of the invention to provide an efficient signal coding scheme that would allow high speed data transmission over multiple pairs of UTP-5 cable. It is a further object of the invention to provide a mechanism to detect the end of the data frame being transmitted, avoiding the overhead of using redundant symbols for carrying such end of frame indications. It is yet a further object of the invention to provide a mechanism to support the carrying of control information, avoiding the overhead of using redundant symbols for carrying such control information. Finally, it is an object of the invention to provide a mechanism to allow pattern substitution for runs exceeding the length of typical clock recovery circuits.